This test has been deadlocking and causing problems on the bots basically since
its inception. Some memory safety issues were fixed in 987dc84b, but the
deadlocks remained afterwards unfortunately.
After some investigation, I've concluded that this is just a situation where OSX
is not guaranteed to run destructors. The fix in 987dc84b observed that OSX was
rewriting the backing TLS memory to its initial state during destruction while
we weren't looking, and this would have the effect of canceling the destructors
of any other initialized TLS slots.
While very difficult to pin down, this is basically what I assume is happening
here, so there doesn't seem to really be anythig we can do to ensure the test
robustly passes on OSX, so just ignore it for now.
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